Overload current protection device using magnetic impedance element

ABSTRACT

An overload current protection device for cutting off power applied from a power supply to a load ( 3 ) such as a motor by means of a contactor (switch) ( 2 ) at overloading. With the configuration, an element ( 40 ) having a magnetic impedance effect as current detection units ( 4   a,    4   b,    4   c ) is used to thereby expand a current detection range by eliminating magnetic saturation due to a core, a problem with a conventional current transformer, thereby providing at low costs an overload current protection device having a wide current detection range.

TECHNICAL FIELD

The present invention relates to an overload current protection devicefor detecting a current flowing through a conductor, and cutting off thecurrent when the current exceeds a predetermined threshold, for example,an overload current protection device capable of controlling the supplyof power to a motor.

BACKGROUND ART

Normally, for example, an overload current protection device of thistype detects that the current flowing to a 3-phase motor through acontactor exceeds a safe threshold, and cuts off the current to themotor depending on the detection result, which has been realized bymaking all or a part the current of the motor flow through a bimetallicelement. That is, if a current is applied to the switch made of abimetal, the bimetal is heated depending on the current magnitude, andthe motor current exceeds a safe threshold for a predetermined time,then the bimetal is bent by heat and a switch connection point is set inan OFF state, and the current supply to the control input of thecontactor is stopped. However, in the system using the switch, it isdifficult to adjust a current when the switch enters the OFF state, andthere arises the problem that the system is in the misarrangement statefor a long time.

To solve the above-mentioned problem, it has been possible toelectronically perform the functions which have conventionally beenrealized by a bimetallic switch. A reliable and easily adjusted devicecan be provided using electronic equipment. However, since thiselectronic system requires a complicated circuit, and a number of partssuch as a constant voltage power source, etc. are required to operate acontact system by appropriately detecting a current. Furthermore, acurrent detection transformer (what is called a CT) is used as currentdetection unit, which causes the problem that a wide current detectionrange cannot be obtained because the iron core generates magneticsaturation. There also is a method of using a magnetoresistive element.However, since it has low sensitivity, it requires an iron core, therebyfailing in obtaining a wide current detection range as in the case ofthe above-mentioned CT. Additionally, since the magnetoresistive elementhas a large fluctuation depending on the temperature and a largedifference among the elements, and is subject to an influence ofdisturbance noise, there occurs the problem that high-precision devicemeans requires a high cost.

Therefore, the present invention aims at providing a low-cost andhigh-precision overload current protection device capable of expanding acurrent detection range without a constant voltage power source, etc.,and without degradation in precision by an environmental characteristicsuch as disturbance noise, etc. and a change with time.

DISCLOSURE OF INVENTION

To solve the above-mentioned problems, in one embodiment of theinvention an overload current protection device, which has a switch forsupplying or cutting off a current from a power source to a load, acurrent detector for detecting the current, and a control power sourcefor applying power to each unit of the device, and cuts off the supplyof the current to the load when an overcurrent occurs, configures thecurrent detector by a magnetic impedance element having a magneticimpedance effect, and detects magnetic flux generated by a current usingthe magnetic impedance element.

In another embodiment of the invention, an overload current protectiondevice, which has a switch for supplying or cutting off a multiphasecurrent from a power source to a load, a plurality of current detectorsfor detecting the multiphase current for each phase, and a control powersource for applying power to each unit of the device, and cuts off thesupply of the current to the load when an overcurrent occurs, configureseach of the plurality of current detectors by a magnetic impedanceelement having a magnetic impedance effect, and detects magnetic fluxgenerated by a current using the magnetic impedance element.

One of the embodiments of the invention has wiring for leading thecurrent and a substrate for fixing the wiring, wherein the magneticimpedance element is arranged near the wiring on the substrate so thatthe magnetic flux generated by a current can be directly detected by amagnetic impedance element.

In another embodiment of the invention, the device further includes: acurrent applying unit for applying a high frequency current to themagnetic impedance element; a detection unit for detecting an output ofthe magnetic impedance element; a correction unit for correcting adetection result; a magnetic field applying unit for applying a biasmagnetic field to the magnetic impedance element; a magnetic fieldvariable unit for changing a median value of the bias magnetic field;and a control unit for controlling a change of the median value. Withthe configuration, the median value of the bias magnetic field ischanged and the output is detected, and the output can be correcteddepending on the detection result.

In an embodiment of the invention, the magnetic field applying unit canbe configured by a bias coil and an oscillation unit.

In another embodiment of the invention, the magnetic field variable unitcan be configured by an offset coil and a constant current generationunit, or the magnetic field variable unit can be configured by aconstant current generation unit, a switch unit, and an addition unit,wherein a constant voltage can be applied to the bias coil.

In yet another embodiment of the invention, two magnetic impedanceelements are arranged in the positions in which the absolute values ofthe output in response to the magnetic flux generated by a current canbe equal and the polarity can be opposite to each other, and the currentcan be detected from the calculation result of the difference betweenthe outputs of the two magnetic impedance elements, or two magneticimpedance elements are arranged in the positions in which the absolutevalues of the output in response to the magnetic flux generated by acurrent can be equal and the polarity can be the same, and the currentcan be detected from the calculation result of the difference betweenthe outputs of the two magnetic impedance elements.

In an embodiment of the invention, a shield for cutting off an externalmagnetic field can be provided for the wiring for leading the currentand the two magnetic impedance elements.

In another embodiment of the invention, the control power source can beconfigured by a power supply transformer having primary winding insertedin a current supply path from the power source to a load and secondarywinding electrically connected to the primary winding, a capacitor forstoring the current of the secondary winding of the power supplytransformer, and a voltage adjuster.

In yet another embodiment of the invention, the control power source canbe configured by a power supply transformer having plural pieces ofprimary winding, each primary winding for each phase being wound aroundan iron core, inserted in a current supply path from the power source toa load and secondary winding, a capacitor for storing the current of thesecondary winding of the power supply transformer, and a voltageadjuster, and the turns of the primary winding of each phase can bedifferent among the phases.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the first embodiment of the presentinvention;

FIG. 2 is a plan view of an example of the configuration of the MIelement unit shown in FIG. 1;

FIG. 3 is a block diagram showing an example of a detection circuitshown in FIG. 1;

FIG. 4 shows the circuit of a practical example of the constant currentcircuit shown in FIG. 3;

FIG. 5 shows the method for detecting the output sensitivity of an MIelement;

FIG. 6 is a block diagram showing another example of a detectioncircuit;

FIG. 7 is a plan view showing another example of the configuration of anMI element unit;

FIG. 8 shows the circuit of a practical example of the oscillationcircuit for bias shown in FIG. 6;

FIG. 9 is an oblique view of an example of another configuration of anMI element unit;

FIG. 10 is an explanatory view showing the influence of a currentflowing through the adjacent wiring shown in FIG. 9;

FIG. 11 is a plan view showing an example of the configuration of themagnetic shield of an MI element unit;

FIG. 12 is a block diagram showing another example of a detectioncircuit;

FIG. 13 is a block diagram showing the second embodiment of the presentinvention; and

FIG. 14 shows the configuration of another example of a control powersource unit.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows the configuration of the system showing an embodiment ofthe present invention.

Reference characters R, S, and T denote power supply lines connected toa 3-phase AC power source not shown in the attached drawings, and areconnected to a motor 3 through a 3-phase contactor (switch) 2 and threepower supply transformers 5 a, 5 b, and 5 c. Current detectors 4 a, 4 b,and 4 c are arranged for each phase between the 3-phase contactor 2 andthe three power supply transformers 5 a, 5 b, and 5 c. The contactor 2includes contact points 2 a, 2 b, and 2 c each of which is coupled tothe motor 3 through the primary winding of respective power supplytransformers 5 a, 5 b, and 5 c by the different power supply lines. Theset of contact points is mechanically coupled for simultaneous operationby an electromagnetic coil 2 d. The electromagnetic coil 2 d isconnected to the digital output of a microcomputer 8. A control circuitincluding the microcomputer 8, the current detectors 4 a, 4 b, and 4 c,the power supply transformers 5 a, 5 b, and 5 c, etc. form an electronicoverload relay 1.

The output of the current detectors 4 a, 4 b, and 4 c is sequentiallyswitched through a switch 6. The output of the current detectors 4 a, 4b, and 4 c selected by the switch 6 is connected to the analog input ofthe microcomputer 8 through a half-wave rectifier 7

The control power source is connected to a first capacitor C0 throughrectifier diodes D0, D1, and D2 from the secondary winding of the powersupply transformers 5 a, 5 b, and 5 c, and is then formed. The firstcapacitor C0 is connected between the positive input of a voltageadjuster 9 and the ground, and a second (stabilizing) capacitor C1 isconnected between the positive output of the voltage adjuster 9 and theground so that a voltage Vcc at a constant level can be provided as acontrol power source from the voltage adjuster 9. Reference numeralsD3,D4, and D5 denote protective diodes.

The practical configuration of the current detectors 4 a, 4 b, and 4 cconfigured by a current detection element 40 and a detection circuit 41is described below by referring to FIGS. 2 and 3. Since theconfigurations of the current detectors 4 a, 4 b, and 4 c are the same,one of them is described below as a representative configuration.

In FIG. 2, a magnetic impedance element (MI) element 40 has a magneticimpedance effect, and wiring 200 leads a current of a certain phase.Reference numeral 401 denotes a bias coil, reference numeral 402 denotesan offset coil, and reference numeral 403 denotes a bobbin. The MIelement 40 can be, for example, an amorphous wire as disclosed byJapanese Patent Application Laid-open No. Hei 6-281712, or a thin filmas disclosed by Japanese Patent Application Laid-open No. Hei 8-330645.

FIG. 3 shows an example of a detection circuit.

In FIG. 3, an oscillation circuit 411 applies a high frequency currentto the MI element 40, an oscillation circuit (or a constant currentcircuit) 412 drives the bias coil 401, a constant current circuit (forbias shift) 413 drives the offset coil 402, a control circuit 414controls the presence/absence, etc. of an offset for a bias, referencenumerals R1 and R2 denote resistors, reference numeral C2 denotes acapacitor, thereby forming a drive unit.

On the other hand, the detection unit is configured by a detectioncircuit 415, holding circuits 416 a and 416 b, a differentialamplification circuit 417, etc.

With the above-mentioned configuration, the oscillation circuit 412drives the bias coil 401, the constant current circuit 413 drives theoffset coil 402, and the oscillation circuit 411 applies a highfrequency current to the MI element 40, thereby changing the impedanceof the MI element 40. The change is detected by the detection circuit415 in the detection circuit 41, the holding circuits 416 a and 416 brespectively hold the plus (+) side and the minus (−) side of a detectedwave, and the difference is detected by the differential amplificationcircuit 417.

As shown in FIG. 4, the constant current circuit for driving an offsetcoil is configured by, for example, a constant current circuit CC and acurrent mirror CM. When the reference voltage of a constant voltagediode ZD is represented by Vref, and the resistance value is representedby Rref, the current I applied to the offset coil 402 is obtained by thefollowing equation.I=Vref/Rref

FIG. 5 shows the method for detecting the output sensitivity in the MIelement 40 in which an external magnetic field is zero, and an AC biasis applied.

In the case 1 shown in (a) and (b) in FIG. 5, the median value of thebias magnetic field is a magnetic field of zero, and the output of theholding circuits 416 a and 416 b is equal to each other, and the outputof the differential amplification circuit 417 is zero.

In the case 2 shown in (c) and (d) in FIG. 5, the median value of thebias magnetic field is shifted by ΔH. As a result, the output differencebetween the holding circuits 416 a and 416 b is ΔV, and the output ofthe differential amplification circuit 417 is α×ΔV (α indicates a gainof the differential amplification circuit). Therefore, the sensitivityof the magnetic sensor (MI element) can be represented by ΔV/ΔH.

This indicates that the detection sensitivity of the magnetic sensor (MIelement) can be automatically detected by obtaining the output voltageby changing the median value of the bias magnetic field by the knownvalue for the magnetic field. Therefore, although the detectionsensitivity of the sensor changes by an environmental variance, a changewith time, etc., the detection sensitivity of a sensor can be obtainedby the method shown in FIG. 5, and a correction can be automaticallymade.

The external magnetic field is zero in the description above. However,when an arbitrary magnetic field is applied, the detection sensitivityof the magnetic sensor obtained when the median value of the biasmagnetic field is changed by the known value for the magnetic fieldremains unchanged as compared with the detection sensitivity shown inFIG. 5 only by an offset for the applied magnetic field. Although the ACbias is applied in the above-mentioned case, the detection sensitivityof a sensor can also be automatically detected and corrected by applyinga DC bias.

FIG. 6 shows another example of the detection circuit.

In FIGS. 2 and 3, an offset coil is used to apply an offset magneticfield for changing the median value of the bias magnetic field. Thisexample is characterized in that the direct current of the oscillationpulse for driving the bias coil is changed. The constant current circuitfor bias shift is omitted in and after FIG. 6, and the offset coil shownin FIG. 2 is omitted in the magnetic sensor in and after FIG. 7.

Therefore, as shown in FIG. 8 for example, the oscillation circuit forbias 412 a shown in FIG. 6 is configured by an oscillation circuit OS ofseveral tens KHz, a constant voltage circuit CV, a switch SW, and anaddition circuit AD. Since a switch SW is normally grounded at thepotential of zero, the pulse from the oscillation circuit OS and theoffset voltage from the constant voltage circuit CV is added in theaddition circuit AD when the switch SW is connected to the constantvoltage circuit CV by a control circuit 414 a although the offset amountfrom the oscillation circuit OS is zero. As a result, the offsetmagnetic field which changes the median value of the bias magnetic fieldis applied.

Since the automatic detection and correction of the detectionsensitivity of a sensor is performed similarly as in the case shown inFIGS. 2 and 3, the explanation is omitted here.

The system using one MI element is described above, but there can be twoor more MI elements as described below. In the explanation below, a biascoil, etc. is omitted, but it is obvious that the bias coil, etc. can beused.

FIG. 9 shows an example of arranging two MI elements. Reference numerals40 a and 40 b shown in FIG. 9 denote MI elements. Reference numeral 200denotes wiring for leading a current of a certain phase. Referencenumeral 300 denotes a substrate for fixing the wiring 200 and the MIelements 40 a and 40 b. The reference numeral 41 denotes a detectioncircuit.

FIG. 10 is an explanatory view of the influence of the current flowingthrough adjacent wiring with the configuration shown in FIG. 9, andshows the case in which a current I1 and another current I2 flowadjacent to each other. Assuming that the magnetic flux generated by thecurrents I1 and I2 are represented by φ1 and φ2 respectively, and theoutput levels appearing on the two MI elements by φ1 and φ2 are S2 andN3 respectively, the output of the difference between the two MIelements 40 a and 40 b is calculated as follows.

$\begin{matrix}{{{differential}\mspace{14mu}{output}} = {{{{output}\mspace{14mu}{of}\mspace{14mu} 40a} - {{output}\mspace{14mu}{of}\mspace{14mu} 40b}}\mspace{194mu} = {{{S2} + {N3} - \left( {{- {S2}} + {N3}} \right)}\mspace{194mu} = {2 \times {S2}}}}} & (1)\end{matrix}$thus enabling the detection of the current I1 without the influence ofthe current I2 of adjacent wiring 210. Furthermore, when a uniformexternal magnetic field is applied as noise, the output of equal sizeand sign appears on the two MI elements. Therefore, the influence of theexternal magnetic field can be canceled as in the case of the adjacentwiring.

FIG. 11 shows the configuration of another example of removing theinfluence of a current flowing through adjacent wiring

In this example, as compared with that shown in FIG. 11, a shield plate404 is added as a magnetic shield using Permalloy, etc. That is, asshown in FIGS. 9 and 10, the influence of a current flowing throughadjacent wiring can be logically canceled, but the noise of an externalmagnetic field cannot be completely canceled due to the variance insensitivity between the two MI elements, the influence of thedisplacements, etc. Therefore, the influence can be reduced by themagnetic shield.

FIG. 12 shows another example of a detection circuit.

The detection circuit 41 applies a high frequency current to the MIelements 40 a and 40 b using an oscillation circuit 411 a, and thepartial pressure resistors R3 and R4, detects a change in impedance bythe magnetic field of the MI elements 40 a and 40 b as a change involtage using detection circuits 415 a and 415 b, generates the outputproportional to the difference between the MI elements 40 a and 40 busing a differential circuit 417 a, and retrieves the output using anamplification circuit 418. The differential circuit 417 a can bereplaced with an addition circuit, and the output proportional to thedifference between the MI elements 40 a and 40 b can be replaced withthe output proportional to the sum of the MI elements 40 a and 40 b.

In the above-mentioned example, the magnetic field detection directionsare the same between the two MI elements. However, it is obvious that acurrent can be detected without the influence of the disturbance noiseas in the above-mentioned example by obtaining a sum of the output ofthe two MI elements with the magnetic field detection directions setopposite each other.

FIG. 13 shows another embodiment of the present invention.

In FIG. 1, the power supply transformer is provided for each phase.However, in FIG. 13, one core 53 is provided with primary winding 51 a,51 b, and 51 c for each phase, and power is applied from secondarywinding 52 through the diode D6. Reference numeral D7 denotes aprotective diode.

FIG. 14 shows an example of using a toroidal core. In this case, therate of turns of the primary winding 51 a, 51 b, and 51 c is selectedsuch that an appropriate current level can be provided from thesecondary winding 52. That is, since no magnetic flux can be generatedwhen the turns of the primary winding is equally set and balanced, theturns are set different from each other.

The 3-phase alternating current is applied in the above-mentionedexample. However, it is obvious that the single-phase alternatingcurrent can also be appropriately used by considering that itcorresponds to one phase of the 3-phase alternating current.

INDUSTRIAL APPLICABILITY

According to the present invention, the following advantages can beachieved.

1) Since a magnetic detection element having a magnetic impedance effectis used, the magnetic saturation by a iron core caused by a widely usedcurrent detection transformer does not occur, thereby widening thecurrent detection range.

2) Using a magnetic impedance element, changing the median value of thebias magnetic field by a known amount of magnetic field, and detectingthe output voltage enable the sensitivity of a sensor to beautomatically detected. As a result, a correction can be automaticallymade with the detection sensitivity of a sensor changed by anenvironmental characteristic and a change with time. Therefore, a highprecision device can be provided without degradation in precision by anenvironmental characteristic and a change with time.3) Since it is not necessary to externally provide a control powersource, the device can be appropriately used as a general-purpose deviceat a reduced total cost.4) When a control power source is used with a multiphase AC powersource, it is not necessary to provide the power supply transformer foreach phase, that is, at least one power supply transformer can beprovided. Therefore, the number of necessary units can be smaller, andthe entire cost can be reduced.5) If the absolute value of the output of the magnetic detection elementcan be the same as the value of the magnetic flux generated by thecurrent, and the difference between two elements can be detected in thepositions in which the opposite polarity can be detected, then theinfluence of the magnetic field by an external magnetic field and acurrent flowing through the adjacent wiring can be eliminated, and adevice excellent in environmental resistance can be provided.

1. An overload current protection device which cuts off the supply ofcurrent to a load when an overcurrent occurs, comprising: a switch forsupplying or cutting off the current from a power source to the load; acurrent detector for detecting the current; and a control power sourcefor applying power to each unit of the device, wherein the currentdetector is configured by a magnetic impedance element having a magneticimpedance effect, and magnetic flux generated by the current is detectedusing the magnetic impedance element, the overload current protectiondevice further comprising: a current applying unit applying a highfrequency current to the magnetic impedance element; a detection unitdetecting an output of the magnetic impedance element; a correction unitcorrecting a detection result; a magnetic field applying unit applying abias magnetic field to the magnetic impedance element; a magnetic fieldvariable unit changing a median value of the bias magnetic field; and acontrol unit controlling a change of the median value, wherein themedian value of the bias magnetic field is changed, the output of themagnetic impedance element is detected, and the output is correcteddepending on the detection result.
 2. An overload current protectiondevice which cuts off the supply of current to a load when anovercurrent occurs, comprising: a switch for supplying or cutting off amultiphase current from a power source to a load; a plurality of currentdetectors for detecting the multiphase current for each phase; and acontrol power source for applying power to each unit of the device,wherein each of the plurality of current detectors is configured by amagnetic impedance element having a magnetic impedance effect, andmagnetic flux generated by the current is detected using the magneticimpedance element, the overload current protection device furthercomprising: a current applying unit applying a high frequency current tothe magnetic impedance element; a detection unit detecting an output ofthe magnetic impedance element; a correction unit correcting a detectionresult; a magnetic field applying unit applying a bias magnetic field tothe magnetic impedance element; a magnetic field variable unit changinga median value of the bias magnetic field; and a control unitcontrolling a change of the median value, wherein the median value ofthe bias magnetic field is changed, the output of the magnetic impedanceis detected, and the output is corrected depending on the detectionresult.
 3. An overload current protection device which cuts off a supplyof current to a load when an overcurrent occurs, comprising: a switchfor supplying or cutting off the current from a power source to theload; a current detector for detecting the current; and a control powersource for applying power to each unit of the device, wherein thecurrent detector is configured by a magnetic impedance element having amagnetic impedance effect, and magnetic flux generated by the current isdetected using the magnetic impedance element, wiring for conducting thecurrent and a substrate for fixing the wiring are provided, and themagnetic impedance element is arranged near the wiring on the substrateso that the magnetic flux generated by the current can be directlydetected by the magnetic impedance element, the overload currentprotection device further comprising: a current applying unit applying ahigh frequency current to the magnetic impedance element; a detectionunit detecting an output of the magnetic impedance element; a correctionunit correcting a detection result; a magnetic field applying unitapplying a bias magnetic field to the magnetic impedance element; amagnetic field variable unit changing a median value of the biasmagnetic field; and a control unit controlling a change of the medianvalue, wherein the median value of the bias magnetic field is changed,the output of the magnetic impedance element is detected, and the outputis corrected depending on a detection result.
 4. The device according toclaim 3, wherein the magnetic field applying unit is configured by abias coil and an oscillation unit.
 5. The device according to claim 4,wherein the magnetic field variable unit is configured by an offset coiland a constant current generation unit.
 6. The device according to claim4, wherein the magnetic field variable unit is configured by a constantcurrent generation unit, a switch unit, and an addition unit, and aconstant voltage is applied to the bias coil.
 7. The device according toclaim 3, wherein the magnetic field variable unit is configured by anoffset coil and a constant current generation unit.
 8. The deviceaccording to claim 3, wherein the magnetic field variable unit isconfigured by a constant current generation unit, a switch unit, and anaddition unit, and a constant voltage is applied to the bias coil.